1. Field of the Invention
The present invention relates to an inkjet ink composition, and more particularly relates to a low odor environmentally-friendly nano-pigment inkjet ink composition applied to the solvent-based piezo inkjet printer. The inkjet ink composition contains low odor and low toxic solvent but without containing any hazardous substances. The inkjet ink composition has good storage stability and inkjet stability, and the printed image has the properties of high gloss, abrasion resistance, and weather resistance, and also the printhead nozzles are not easily clogged by such an inkjet ink composition. The inkjet ink composition can meet both the environmental requirement and the print quality requirement.
2. The Prior Arts
Digital inkjet printer is controlled by a software system, in which the ink is ejected onto the printing substrate to form the words or the images. Nowadays, two fundamentally different types of printheads for inkjet printing are piezoelectric and thermal-bubble types based on the working mechanism of the nozzles. Commonly, piezoelectric inkjet printheads include one or more fluid chambers, engineered to deform during the application of an external voltage. Typically, this deformation decreases the chamber's volume, which causes a droplet of ink to be ejected through the nozzles at one end of the fluid chamber, and the ejected ink droplets can be precisely controlled to have the desired shapes and the right flying directions. Strong shear force will be produced when the ink goes through the narrow fluid chamber, and thereby the ink with low viscosity is required so that the ink commonly contains a large amount of solvent. The inkjet inks can be broadly divided into water-based inks, oil-based inks, and UV curable inks The oil-based pigment ink is directly ejected onto polyvinyl chloride (PVC) material and applied to the outdoor large advertising boards, and the oil-based pigment ink can be divided into solvent-based ink, mild solvent-based ink, and ECO-solvent-based ink.
The solvent-based inks are usually very corrosive, which allows them to penetrate uncoated PVC substrate, and thereby have good adhesion to the substrate, and the printed images have high resolution. The solvents used in the solvent-based inks include the strongly corrosive ketones such as cyclohexanone, isophorone, and thus such solvents are hazardous for human and environment.
The mild esters are used in the mild solvent-based inks stead of ketones, and the mild esters include ethylene glycol monobutyl ether acetate and diethylene glycol monobutyl ether acetate. The above esters have the following advantages: good dispersion to pigment, short grinding time, making pigment to have good color development performance and high gloss, and thus they become the first choice to disperse the pigment ink. However, the above esters have the disadvantages of fast evaporation rate and strong odor. U.S. Pat. No. 5,371,279 disclosed that ethylene glycol monobutyl ether acetate will be decomposed into acidic substances under high humidity environment. US Pat. App. No. 2009/0,139,432 A1 disclosed that alcohol amine was added for preventing the formation of the acidic substances which can cause the corrosion of nozzles, and consequently the ink miss-ejection and oblique ejection will occur, and thereby the printing quality is poor. EP Pat. No. 2284226 A1 disclosed the corrosion problem for an ink, and cyclohexylamine and imidazole as acidity regulator are used, but they are toxic and corrosive, which can cause the operation safety problem.
Glycol ethers (such as diethylene glycol diethyl ether) are used in the environment-friendly inks as main solvent. The glycol ethers have the advantages of low toxicity, no odor and high security, and have the disadvantages of low boiling point, the formation of a large amount of volatile organic compound (VOCs) gases, low polarity, poor solubility for polymer, and poor dispersivity. In addition, the serious bleeding phenomenon and the blockage of the nozzles occur due to the low surface tension of the solvent. Because of no corrosion to the PVC material, the above solvents must be used with the other appropriate solvents or additives to improve the surface tension of the ink and corrosion to the substrates. WO Pat. App. No. 2010/133889A2 disclosed that 25-45% of propylene carbonate was added to glycol ether to reduce the amount of the volatile organic compounds in the ink, but if propylene carbonate is added over a certain limit, the drying rate of the printed image will become slow so that the printing speed cannot be improved. U.S. Pat. No. 7,125,447B2 disclosed that the addition of the γ-butyrolactone solvent to the glycol ether solvent contained in the oil-based inkjet ink can improve the penetration to the PVC material, and increase the leveling and the drying rate of ink. However, according to the latest CLP/GHS Annex VI to Regulation (EC) No. 1272/2008, the safety of γ-Butyrolactone is changed from the original warning label of R22 (harmful by inhalation) and R36 (eye irritation) to R22 (harmful by inhalation) and R41 (serious eye injury) and R67 (vapors may cause drowsiness and dizziness). US Pat. App. 2005/0148689A1 disclosed that glycol ether solvent with 1-30% of N-methyl pyrrolidone or N-ethyl pyrrolidone has low odor and is easily soluble to the PVC material. According to the EU REACH regulation, N-methyl pyrrolidone appears to affect the fecundity of females and is identified as substances of very high concern (SVHC). Furthermore, the nozzles can be dissolved and damaged by a large amount of high polar pyrrolidone solvents. The PVC material can be highly corroded by the ketone and ester solvents, and the ketone and ester solvents have heavy odor and low safety. Moreover, with the change of chemical safety regulations, it becomes very difficult to choose an appropriate solvent for improving the safety and the resolution of the ink.